1. Technical Field
The present invention particularly relates to a magneto-optical recording medium for which at least one of recording and reproduction is conducted by irradiating a light onto the magneto-optical recording medium, an optical head for conducting at least one of recording and reproduction by irradiating a light and a recording and reproducing apparatus comprising the optical head.
2. Description of the Related Art
FIG. 16 is a schematic cross-sectional view of an example of a conventional magneto-optical recording medium for which recording or reproduction is conducted by irradiating a light onto the medium and a manner of light irradiation.
In this conventional magneto-optical recording medium 100, the first dielectric layer 102 made of, for example, SiN and the like, an information recording layer 103 consisting of a magnetic film made of, for example, TbFeCo and the like, the first dielectric layer 104 made of, for example, SiN and the like and a reflection layer 105 made of, for example, Al and the like are sequentially provided on a substrate 101. A protective layer 107 made of, for example, ultraviolet hardened resin and the like is formed on the reflection layer 105.
An information signal is written in the magneto-optical recording medium 100 shown in FIG. 16 in the magnetization direction of the magnetic film 103. When an information signal is recorded and reproduced, an incidence of a laser light L is carried out from the substrate 101 side as shown in FIG. 16.
In case of reading the signal recorded on the magneto-optical recording medium 100 shown in FIG. 16, a laser light L is incident on an objective lens 124 from a laser light source 121 through a collimator lens 122 and a beam splitter 123 in a recording and reproducing apparatus 120 as shown in FIG. 17.
Here, it is assumed that the laser light incident on the objective lens 124 is a linear polarized light, whose polarization direction is shown in FIG. 18.
The laser light L incident on the objective lens 124 is converged onto the information recording layer 103 of the magneto-optical recording medium 100 by the objective lens 124.
The convergent light is reflected and the polarization state of the reflected light is changed by the Kerr effect from the information recording layer 103.
The polarization direction of the reflected light is shown in FIG. 19.
As shown in FIGS. 18 and 19, the polarization direction of a return light reflected by the recording medium and returned from the objective lens 124 varies depending on a magnetization direction according to the information recorded on the information recording layer 103.
As shown in FIG. 17, the return light is passed through the objective lens 124 again, incident on the beam splitter 123, reflected by the beam splitter 123 and fetched.
The return light reflected by the beam splitter 123 and fetched is first incident on a half-wave plate 125 and the polarization direction of the return light is rotated.
Next, the return light is incident on and split by a polarization beam splitter 126 into two polarization components having polarization directions perpendicular to each other. Among them, the polarization component transmitted by the polarization beam splitter 126 is detected by the first photo-detector 127 and that reflected by the polarization beam splitter 126 is detected by the second photo-detector 128.
In such a system for recording and reproducing the information on a magneto-optical recording medium, it is effective to increase the numerical aperture N. A. of the objective lens for converging a laser light L used for recording and reproduction, to thereby reduce the spot diameter of the light converged by the objective lens and to enhance resolution for the purpose of increasing recording density.
Here, it is assumed that the spot diameter of the light converged by the objective lens is generally expressed as .lambda./N. A. where .lambda. is the wavelength of a laser light used for recording and reproduction and N. A. is the numerical aperture of the objective lens.
The numerical aperture N. A. of the objective lens is expressed as n.sub.0.multidot.sin.theta. where n.sub.0 is the refractive index of a medium and .theta. is the angle of the peripheral light beam of the objective lens. Obviously, therefore, if air is the medium (i.e., n.sub.0 =1), the numerical aperture of the objective lens N. A. does not exceed 1.
As a technique for exceeding the limit, there is proposed a recording and reproducing apparatus using a solid immersion light. The solid immersion lens is supported by and opposite to the magneto-optical recording medium at a distance of less than the wavelength of a light used for recording and reproduction. In the recording and reproducing apparatus using the solid immersion lens, a convergent light is incident on the solid immersion lens and most parts of the incident beams are totally reflected on the end face of the solid immersion lens. Utilizing a so-called evanescent light effused from the end face of the solid immersion lens, the information is recorded on and reproduced from the magneto-optical recording medium. At this time, if a medium having a refractive index n.sub.0 of greater than 1 is used for the solid immersion lens, the numerical aperture N. A. can be made not less than 1.
However, as stated above, if information is recorded on and reproduced from the magneto-optical recording medium using an evanescent light, the light propagated into the magneto-optical recording medium is different in property from a light conventionally applied to information recording or reproduction by the magneto-optical recording medium. Due to this, for example, enhancement conditions of a multilayer structure combined with a transparent dielectric cannot be applied or the information on the phase of the light differs from the incidence direction when a linear polarized light is incident on the medium. As a result, the quality of the reproduced signal disadvantageously deteriorates.